1. Field of the Invention
The present invention relates to a system and method for obtaining a higher dynamic range in images. More particularly, the present invention relates to enhancing the signal dynamic range in imaging systems for video cameras, digital cameras, and solid-state electronic imaging devices, which utilize charge-coupled device (CCD) imagers, CMOS imagers, or other types of imagers.
2. Description of Related Art
A video camera typically contains a solid-state electronic imaging device such as a CCD (charge-coupled device) or CMOS imager for generating a video signal representing an image of a subject. Typical CCDs have a signal-to-noise ratio (SNR) of below 60 dB at a particular exposure setting. Because the CCD exposure may be changed only on a per frame (or slower) basis, the dynamic range within a single image is limited to the 60 dB range. By carefully designing the correlated double sampling circuitry (CDS) and automatic-gain-control circuitry (AGC), and by using a low noise analog-to-digital converter (ADC), the noise in the output will tend to be dominated by the CCD noise and not that of the CDS, AGC and ADC electronic circuitry. Therefore, 10-bit ADCs are adequate for typical CCD applications.
A 10-bit ADC, however, limits the dynamic range resulting in poor contrast and color depth in certain pictures or visual fields. For example, when the difference in luminance between a bright portion and a dark portion included in a visual field is large, it is difficult to adjust the gain to pick up both the dark portion and the bright portion under proper exposure conditions. If the gain is adjusted to the dark portion, the bright portion appears washed out. On the other hand, if the exposure conditions are adjusted to the bright portion, the dark portion appears black.
Conventional AGCs include control circuitry that attempts to adjust automatically the analog gain applied to the analog image signal to increase the analog signal to use the maximum range of the ADC, which is digitizing the analog image signal. This automatic adjustment is generally based upon various image parameters that are "averaged" over a single frame or multiple frames. Once the analog image signal has been digitized by the ADC, the AGC will generally also determine whether the analog gain adjustment has been effective to utilize the full signal processing range. If not, a digital gain is applied to the digitized image data to increase the digital output signal to the full range of the DSP circuitry. In this way, conventional AGCs attempt to provide per-frame or per-multiple-frame automatic gain adjustment of both analog and digital gains to increase the brightness of the image as a whole.
Conventional AGCs, however, are ineffective in high contrast situations. For example, the contrast and color depth problem discussed above becomes particularly difficult to overcome where a subject in the image is inside a room and in front of a very bright background, such as a window on a bright day. Conventional AGCs are unable to improve such high contrast images because the AGC control loop typically calculates an "average" gain over a full frame or multiple frames, and then applies the gain value uniformly to all of the pixels in the image frame. In this scheme, if the analog gain is increased to improve the low intensity portions of the image, the brighter pixels will saturate and appear washed out. On the other hand, if the analog gain is reduced to get greater contrast in the brighter portions of the image, the low intensity portions of the image will get even darker and appear black.
To solve this contrast problem, prior approaches have generally been to adjust the exposure control of the CCD to be the best possible contrast or to increase the luminance of a main subject by strobe flashing. Another prior technique has been to convert the image signal from the CCD into digital information with an ADC, and then split the image into a bright portion and a dark portion. These two portions may then be processed separately and later recombined before processing the digital pixel information with digital signal processing circuitry (DSP) to provide an analog video output signal. This prior approach, however, is not particularly effective in achieving greater resolution or handling high contrast situations.
What is needed to solve this contrast problem is a new system and approach for providing increased dynamic range without losing image information.